At present the large-volume commercial production of preforms for plastic containers, such as polyethylene terephthalate (PET) beverage bottles, utilizes multiposition robotically-activated cooling tubes to reduce the overall cycle time. A conventional post-mold robotic cooling system receives partially-cooled injection-molded preforms from the mold, cools the preforms down to a temperature sufficient to allow their release without damage onto a conveyor belt or into a large storage container (gaylord), and meanwhile frees the mold cavity for the injection of the next set of preforms. For example, as illustrated schematically in FIGS. 6A-6C, a three-position robotic cooler 100 having 96 cooling tubes 101 may work in conjunction with a 32-cavity injection molding machine 102 by sequentially accepting three sets (I, II, III) of freshly-molded preforms for pose mold cooling. While one set of preforms 103 is received from the mold cores (see FIG. 6B), the previous two sets are cooling in the robot cooling tubes. After remaining in the cooling tubes for two injection mold cycles, the first set of preforms are now released onto the conveyor or into the gaylord, being sufficiently cool that no surface damage or physical distortion (i.e., warpage) occurs by such transfer, and the robotic cooler then returns to accept another set of freshly molded preforms. FIG. 6A illustrates the "mold close" position during injection molding of one set of preforms; FIG. 6B illustrates the "mold open" position wherein the one set of preforms are transferred off the cores and into The cooling tubes; and FIG. 6C illustrates the sequential filling/emptying of the cooling tubes over successive molding cycles.
However, there is a problem optimizing the high-speed robotic cooling production process with multilayer preforms. More specifically, it has been found that there is a tendency for such preforms to delaminate in the cooling tubes. Delamination refers to separation of the multilayer structure at the boundary of different material layers. A delaminated preform is totally unusable and must be discarded, resulting in reduced manufacturing efficiency and higher unit costs. To prevent delamination, efforts have been made to increase the cooling time in the mold, however, this increases the cycle time and reduces the production throughput. Another approach is to increase the cooling tube water temperature, but this also reduces its effectiveness and increases cycle time. Another approach is to provide adhesives between the layers to prevent delamination, but such adhesives render the containers nonrecyclable (i.e., during recycling, the multiple layers of the container must be separated and the materials of the various layers reclaimed). Thus, at present there is no satisfactory solution to this problem.
It would be commercially desirable to provide preforms for smaller size multilayer plastic containers, such as a 16 or 20-ounce single service juice container. However, it is difficult utilizing prior art injection technology to provide such preforms at a cost which can compete with glass bottles or juice boxes (i.e., Tetra-pak). Thus, many small-size beverage containers are still made of glass or multilayer boxes, in spite of the weight and breakage (safety) problems of glass and the nonrecyclability of juice boxes. The problem with present commercial PET preform production is that it is only cost-competitive vs alternative materials for larger size containers. Thus, PET preform manufacturers must increase production and capital efficiency in order to market smaller size containers at a price competitive with glass and juice box alternatives.
Therefore, it would be desirable to provide a method of making multilayer preforms without delamination and in a cost-effective and capital-efficient manner.